The invention relates to a device for stabilising the axis of a variable field guidance system with respect to the axis of a sighting device comprising an objective lens, a reticle and an ocular. The guidance system comprises an optical guidance beam transmitter, a scanning or coding system for the guidance field whose centre is projected by means of a variable focus or zoom objective lens along a collimated beam which is parallel to the guidance axis connecting the optical axis of the zoom lens and the guidance field centre.
This device is particularly used in laser beam missile guidance systems in which the angular guidance field is variable as a predetermined function of the distance of the missile.
Such systems generally use variable focus objective lenses for projecting the guidance beam along the axis defining the missile trajectory. The ratio between the focal lengths of these objective lenses between the instant of launching and the instant of the end of the flight may be higher than 100. These objective lenses are zoom lenses whose focal length is varied by moving several groups of single lens elements along the mechanical axis of the objective lens. One of the principal difficulties of realising these optical systems is to maintain a fixed direction of an optical axis for all focal lengths, which axis must be parallel to the line of sight in all conditions of the environment.
The directional stabilization of the optical axis of zoom lenses may be obtained, for example, by means of a very rigid mechanical system by compensating for all the mechanical tolerances and by minimising the system's sensitivity to temperature variations. This solution leads to heavy and costly systems.